Cave-ins are a very significant safety hazard in mines. Mine shafts sometimes experience cave-ins, collapses, or falling rock due to the layered and stratified makeup of the earth. Frequently, a cave-in is presaged by a measurable sag or subsidence of the roof of the mine shaft. This subsidence may result from the separation of rock layers or other geologic mechanisms and the subsidence of the roof may occur at various depths up into the roof of the mine.
Many prior art devices for monitoring mine roofs are mounted in holes bored into the roof of the mine. These are only capable of monitoring the roof rock up to the depth of the hole, because the measurement being monitored is from the surface of the mine roof to some point on the device mounted in the hole. However, rock layer separation may occur up further in the mine roof than the depth of the hole. This means that a monitoring device that is mounted in a hole will miss the movement, or subsidence, of the mine roof. It is desirable to have a mine roof monitoring apparatus capable of measuring the movement of the mine roof that is not limited to the depth of a mounting hole.
Also, the drilling of the holes in which to mount the monitors is a resource and time consuming process. Mine shafts are confining and not easily accessible. The need to use equipment to drill the holes for mounting the monitors adds a level of work and expense that necessarily reduces the ease and economy of installing the monitors. A more effective and economical means of monitoring mines roofs is needed.
Other prior art devices span between the roof and ceiling to measure ceiling subsidence, but these devices require substantial set-up and some employ electrical measuring techniques. When monitoring a remote location such as a mine shaft or tunnel, ease of set-up is preferred. Similarly, electrical measurements may introduce a degree of complexity not needed for the requirement. Power systems in mines are subject to fluctuations and devices reliant on the power system may generate false positive, while batteries require replacement. Additionally, embodiments of the represent invention need not be monitoring across a strictly vertical distance. Embodiments of the present invention may be employed at angles substantial deviating from vertical, including to a horizontal orientation, however rare monitoring in a horizontal position may be.